Remember the grate where you used the bobble-thing to get the crystal key for the elevator? When you pick up the Sigil of the Mavin (the ring on the corpse after taking the Oath), you're on the other side of that grate. Go there (it's on your automap at this point), pull the lever on the left wall (95% sure it's on the left), and continue out via the elevator just like you did before.
If I understand your question... this is probably the way.
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